The overall objective of this project is to develop approaches for inducing antigen specific tolerance that can be used for transplantation of allografts, including islets. The importance of this goal is that tolerance would allow tissue transplantation without the need for chronic and generalized immunosuppression. Our underlying hypothesis is that if T cells recognize their specific antigen together with death-inducing molecules on a professional antigen presenting cell (APC), then these T cells will be selectively eliminated. Our experimental approach will be to engineer professional antigen presenting cells to express death inducing molecules using pseudotyped retroviral vectors and then to use these transduced cells to tolerize animals. We have three specific Aims. The goal of Aim 1 is to develop constructs and methods for efficiently expressing death-inducing molecules (Fas ligand and TNF) in primary dendritic cells and macrophages. The goal of Aim 2 is to use the "engineered" APCs to induce specific T cell tolerance to major and minor histocompatibility alloantigens prior to transplantation. We will investigate whether this approach can be used to tolerize T cells to alloantigens presented by the direct pathway (on donar APCs) and indirect pathway (on host APCs). We will determine the underlying mechanism by which tolerance is induced. The goal of Aim 3 is to use the "engineered" APCs to tolerize effector and memory T cells (rather than naive cells) to alloantigens or autoantigens. The importance of this Aim is that these cells cause diabetes and other autoimmune diseases and will cause disease to recur in organ transplants. They are also the cells that cause acute rejection. Moreover, such antigen-experienced cells are more difficult to tolerize by other methods. We will determine whether our approach synergizes with CD40L blockade (projects 1 and 2) or other immunosuppressive agents.